This invention relates generally to a process and device for the uninterrupted manufacture of plastic film tubing by blowing up a continuous film tubing by means of an extruder-injector.
Since, with most existing film-blowing devices, the film tubing is extruded vertically upward through an extrusion aperture of the nozzle tip and blowing head, the following disclosure will contain references to "upward" and "downward," etc., for purposes of simplification. It is, however, also possible to have the film tubing extruded vertically downward or to the side, in which case the invention can also be used. Thus, in reference to the invention, "over, above, upward, rising," etc., generally indicate "in the direction of the extrusion, in front of the nozzle tip, flowing", etc., and "under, below, dropping", etc., generally mean "against the direction of the extrusion, behind the nozzle tip, flowing," etc. As with most known film-blowing processes, the film tubing which is extruded vertically upward through the ring-shaped extrusion aperture of the nozzle tip and blowing head, and which is still hot and flexible, is formed by blowing using an inflating medium such as air, for example, which is introduced inside the film tubing by means of the nozzle tip and blowing head. The film tubing then dries instantly. The cooled finished film tubing is pulled up by crushing cylinders and take-up rolls mounted above the nozzle tip, and compressed. The inside of the film tubing is thus sealed below by the nozzle tip and blowing head, and above by the crushing cylinders, so that excess pressure can be generated inside the tubing by the blowing medium introduced. The area between the moldable section of the film tubing below and the solidified section of the tubing above, once the cooled plastic solidifies, is referred to as the freezing zone or frost line. The film tubing section which lies between the nozzle tip and the frost line is referred to as the tube forming zone or expanding zone. In this zone, the plastic film tubing which is still hot is expanded radially and/or stretched axially by the pressure of the blowing medium and/or through the traction speed of the pick-up rolls which is higher than the speed of extrusion of the extrusion aperture, whereby the wall thickness of the extruded film tubing decreases and the desired wall thickness and diameter of the film tubing is thus obtained. The plastic film tubing, here, is supported from the inside through the pressure of the inflating medium, and from the outside through the air pressure which is blown from a ring nozzle surrounding the film tubing and serves both as a bearing pressure and as a cooling medium for the film tubing.
The performance of such a film-blowing device is not limited so much by the performance of the extruder whose speed of extrusion can always be increased, but rather primarily by the cooling power of the cooling device, since during the time it takes for the film tubing to be blown or inflated to the desired thickness, etc., cooling of the plastic at frost line temperature must occur so that further expansion or stretching of the film tubing is prevented by solidification. A reduced inflation time through faster inflation of the film tubing to increase the production capacity of the inflating device can therefore only be obtained if the cooling time can also be reduced correspondingly through improved cooling power. Particularly for the manufacture of relatively thick-walled film tubing such as that used in the manufacture of sacks or containers, for example, it has been attempted to also use an additional inner cooling system inside the extruded film tubing in view of the relatively low heat conductivity of the plastic as compared to the cooled air used for the cooling process which has been previously mentioned.
With one known method used for the inner cooling, the gaseous inflation medium used to blow up the film tubing, such as air for example, is constantly replaced by a continuous flow of carburated fuel. For this, the fuel intake has already been mounted above the nozzle tip at a certain distance from the extrusion aperture, with the fuel outlet disposed at a level with the extrusion aperture, so that the gaseous medium which serves as the cooling and inflating medium flows downward against the direction of the film tubing which is pulled upward. This should permit the carburated fuel to come in contact only with the film which is already partly cooled, and not with the film which is still hot, so as to prevent sudden chilling of the hot film since this would lead to irregular cooling of the tubing wall and thereby result in a poor quality product with uneven wall thickness. However, inner cooling using gas, e.g. air, does have the following disadvantages. In view of the low heat conductivity of the plastic to the cooling gas, large quantities of air are needed for the cooling. Since the inner pressure of the inside of the film tubing can only be slightly above the outer atmospheric pressure, since otherwise the thin plastic film tubing would burst, large quantities of low-pressure air must be supplied and discharged. This requires again large cross-sectional intakes and outlets and a large cross-section of the connecting channels in the nozzle tip inside the ring-shaped or circular extrusion aperture. This requires nozzle tip designs with large openings for the mass current whereby the homogeneity of the plastic is destroyed and the quality of the film produced is minimized. For this reason, on the contrary, attempts have been made to use nozzle tips with smaller openings for the mass current, in accordance with which flow of the plastic in the circular channel of the nozzle is prevented as much as possible from separating into individual branchings, with these branchings being rejoined after a short distance, in order to extrude as much as possible a homgeneous film tubing with uniform wall thickness and quality with the most regular speed of flow possible. However, with the large cross-sectional channels required for the cooling air, the undesirable reacting effect of the cooling air on the nozzle tip can only be controlled to a minimal degree.
To avoid the aforementioned disadvantages, another known inner cooling method for the inside of the film tubing provides for the use of a cooling unit to be mounted onto the nozzle tip. The inflating medium, for example air, inside the film tubing is not replaced but rather is continuously rotated by a fan and thus alternately re-cooled inside the cooling unit. Cooling water, for example, can be fed through the nozzle tip into the exchanger, and then be removed. This method of inner cooling, however, has the following disadvantages. The blowing device drive is made exceptionally difficult by the fact that the section of film which has not yet been blown is very difficult to pull-up over the cooling unit which is mounted above the nozzle tip and blowing head. The electric motor of the fan also develops additional heat which must be eliminated. Humidity as well as vaporizing additives in the plastic, such as plasticizers or lubricants, can condensate on the cooling unit, thereby resulting in quality-reducing irregularities on the surface of the film, due to the condensation dripping onto the hot film and causing local chilling and the steam generated by the possible re-evaporating of the condensation. Dripping condensation on the nozzle can also cause local chilling and thus faults such as the formation of stripes on the film, and the steam generated on the hot nozzle by the evaporating condensation also leads again to defects on the surface of the film. The nozzle tip must be equipped with a cable duct for the current supply to the ventilator motor, with an intake and outlet channel for the heat exchange, and with a channel for the discharge of condensation, thus requiring again the previously mentioned nozzle tip design with undesirably large openings for the mass current. In a short time, the heat transfer is impaired by the dripping plasticizer or lubricant, and the manufacturing process must be interrupted so that the heat exchanger can be cleaned.
The hollow-body blowing process for the manufacturing of bottles, cans and the like according to a completely different technique, differs considerably from the aforementioned film-blowing methods to which the present invention relates. With such process, extruded tubular or pipe-shaped preformed objects are fed intermittently into a blow mold in which they are inflated by means of an inflating medium introduced into the inner space, after which they are cooled. The outer cooling of the hollow body which lies against the blow mold is effected by the blowing device itself which is provided with cooling channels through which a cooling medium can be fed.
With the above hollow-body blowing process, as far as is known, the inner cooling needed in order to create a greater cooling power is not accomplished using air, but rather with a non-gaseous cooling medium such as liquid carbon dioxide which is introduced inside the hollow body at a temperature well below the normal room temperature, inside which it should evaporate. In this case, the cooling medium should not be introduced inside the hollow body until after the blowing process, to prevent the walls of the hollow body from embrittling and to prevent defects on the inside of the hollow body. This sequential blowing and cooling process, however, can only be used for intermittent hollow-body blowing processes, and not for uninterrupted film-blowing processes during which a constant and simultaneous blowing and cooling of the film tubing continuously occurs. Moreover, with the existing hollow-body blowing processes mentioned herein, the carbon dioxide must be prevented from becoming more compact (i.e., resulting in carbonic ice) than carbon dioxide snow, against the inner wall of the hollow body, as this could result in material defects caused by a chilling shock, and for this reason the pressure inside the hollow body must be kept above the triple point of the carbon dioxide; in other words, a pressure above 4.2 atmospheres must be maintained at all times.
This is also possible only with the hollow-body blowing process in which the hollow body rests inside a blow mold, and not with the film-blowing process in which such a high inner pressure would lead to bursting of the thin plastic film tubing. Moreover, as far as the hollow-body blowing process discussed herein is concerned, when using the aforementioned high pressure there is no guarantee that the liquid carbon dioxide which is sprayed will instantly evaporate and that no carbon dioxide drippings will fall on the inner wall of the hollow body, thus causing material defects due to the chilling shock.
With this in mind, it has been suggested that an even more intensive cooling be achieved by using liquid nitrogen or liquid air instead of the carbon dioxide. However, this method has proved unfeasible due to the fact that material damages due to chilling shock could not be avoided.
Another suggestion for the inner cooling process which is necessary when working with the hollow-body blowing process was to spray liquified gas directly into the pre-formed products without prior expansion. It is obvious, however, that this would most definitely lead to the previously mentioned chilling shock and result in the disadvantages listed above.
In order to avoid these disadvantages, it has then been suggested that, for the inner cooling needed with the hollow-body blowing process, the liquified gas used as cooling medium be evaporated at the beginning of the inflating process by letting it flow through warm inlet pipes and a warm blowing mandrel, so that it is introduced in the pre-shaped forms in a gaseous form thus expanding said forms. In further stages of the inflating phase, i.e. following cooling of the pipes and of the blowing mandrel, the cooling medium is still only partly evaporated and finally reaches its liquid state inside the hollow body at the end of the inflating phase. This method should permit the plastic material which is still hot to only come in contact with the gaseous cooling medium, and the liquid or solid cooling medium particles to only enter the hollow body once the latter has been cooled to such a degree that there is no longer a danger that chilling shock and the accompanying material defects could result. The process whereby the liquified cooling medium is first evaporated inside the warm sections of the apparatus until these sections have cooled is also only possible in conjunction with the hollow-body process with which the previously mentioned sections of the apparatus can be warmed up between each individual blowing phase. This method, however, cannot be used in conjunction with the film-blowing procedure with which a constant inner cooling must be effected and the cooling medium must therefore be constantly supplied. For the film-blowing process to which the invention relates, it has also been suggested to use a liquified gas such as carbon dioxide or liquid nitrogen as a cooling medium, for the inner cooling, with the latter being sprayed inside the film tubing. This method, however, also has the disadvantages which were previously mentioned in relation to the existing hollow-body blowing process since mechanical damages can result on the film tubing due to the impact of liquid or solid cooling medium particles or chilling shock, and the accompanying material defects resulting in poor quality material could hardly be avoided.